Phosphorus forms a number of oxoacids, such as hypophosphorus acid, orthophosphorus acid, pyrophosphorous acid, hypo phosphoric acid, orthophosphorus acid, pyrophosphoric acid and metaphosphoricacid acid.
Common and some important features of the structures of the oxoacids of phosphorus:
In all of these oxoacids, the central phosphorus atom is surrounded tetrahedrally by other atoms. At least 1 P=O and 1 p –OH bond are essentially present in all of these oxoacids. P – H bonds are present only in the oxoacids belonging to the “phosphorus acid series”.
Exception: In hypo phosphoric acid, an unusual p – p bond is present.
Only the hydrogen atoms that are attached to oxygen in P – OH bonds are ionisable, and impart acidic character to the compound. Thus, the number of ionisable hydrogens determines the basicity of an acid.
Orthophosphoric acid is a tribasic acid. It has three ionisable hydrogen atoms. Hence, it is a tribasic acid.
Orthophosphoric Hydrogen Ion Phosphate Ion
The P – H bond is not ionisable. The oxoacids of the “phosphorus acid series” have reducing properties because the hydrogen atoms attached directly to the phosphorus atom are not ionisable, and confer reducing properties to the compound. Hypophosphorus acid reduces silver nitrate to metallic silver.
O + H3
→ 4Ag + 4HNO3
Silver nitrate Water Hypophosphorous Metallic Nitric acid Orthophosphoric
acid silver acid
Orthophosphorus acid and pyro phosphorus acid, which belong to the “phosphorus acid series,” are good reducing agents. The oxoacids in lower oxidation states of phosphorus +1& +3 tend to disproportionate to higher and lower oxidation states.
EX: Orthophosphorus acid(oxidation state +3) & Hypophosphorus acid (oxidation state +1) on heating, undergoes disproportionation to form phosphine, which contains phosphorus in the oxidation state -3, and orthophosphoric acid, which contains phosphorus in the oxidation state+5.